Research On Wettability Of Carbon Electrodes For Supercapacitors

In the energy storage fields,supercapacitors have attracted much attention due to their high-power density,long cycling life and so on.Carbon materials with low cost,stable structure and good performance are most widely used supercapacitor electrode materials and have already realized earlier commercialization,but the capacitance needs a further improvement.For carbon-based supercapacitors,the wettability is an unignoring factor which determines the performance of carbon electrodes in electrolytes.This paper is based on three aspects,from the wettability of carbon materials itself,the wettability of carbon electrodes in electrolytes and the influence of binders on wettability of carbon electrodes,designed to investigate the regularity of wettability on electrochemical performance and improvement mechanism.The main content is as follows:The study has been conducted on improving the wettability of activated carbon electrode by adding oxygen-containing functional groups in nitric acid oxidation treatment.Commercial activated carbon YP was used as the active material for nitric acid oxidation treatment.The influence on wettability and electrochemical performance of YP has been discussed.It is found that contact angles of electrodes prepared by activated carbon after 18 h nitric acid oxidation treatment reduced from 124° to 89°,meaning a great improvement of wettability.More electrolytes are accessible to the internal region of electrodes,increasing the effective electrochemical surface area.Besides,oxygen-containing functional groups can provide pseudocapacitance,so that the total capacitance can be improved,up to 30%in alkali electrolyte and 90%in acid electrolytes.However,the conductivity and rate performance were decreased by introducing oxygen-containing functional groups during nitric acid oxidation treatment.Thus,the supercapacitor carbon materials with excellent wettability,high capacitance and good rate performance can be obtained by selecting the appropriate degree of oxidation treatment.The study has been conducted on improving wettability of porous carbon electrodes by the addition of isopropanol to KOH electrolyte.Porous carbons owing differently graphitized degree were prepared with phenolic resin as carbon sources and disodium zinc ethylenediamine tetra acetic acid as activation agents and templates at thermal temperature of 800-1100?.The higher the thermal treatment temperature,the more serious the graphitization degree of porous carbon,leading to worse wettability of carbon material to electrolyte.Isopropanol was added to KOH as an additive to improve the wettability between electrode and electrolyte.Porous carbons with different temperatures were used as active materials to prepare electrodes.It could be found that porous carbon electrode at 1100? has the highest degree of graphitization.The contact angle testing showed that the contact angle of the electrode at 1100? decreased from 1200 to 770 with the addition of isopropanol,with the largest reduction.The electrochemical performance of the supercapacitor with isopropanol was tested under the same conditions.At the current density of 0.1 A g-1,the capacitance of porous carbon electrode at 800? increased by 8%,the capacitance of porous carbon electrode at 950? increased by 15%,and the capacitance of porous carbon electrode at 1100? increased by 27%.By reducing the surface tension,isopropanol enables more ions to enter the interior of the graphitized fringe,which reduces the internal impedance and increases the effective surface,leading to improving the wettability and the electrochemical performance.The study has been conducted on using reduced graphene oxide as a hydrophilic binder to prepare supercapacitor carbon electrodes.That using RGO as a hydrophilic binder instead of traditionally hydrophobic PTFE binder can significantly improve the wettability of the whole electrodes.Comparing with PTFE-bonded electrodes,the contact angle of RGO-bonded electrodes decreased from above 120° to below 60°,meaning a great improvement of wettability.The RGO-bonded electrodes owing a developed three-dimension structure obtained a high specific surface area of 1240 m2 g-1,higher than that of the PTFE-bonded electrodes(1000 m2 g-1).Due to good wettability and developed pore structure,RGO-bonded electrodes show excellent electrochemical performance.In 6 mol L-1 KOH alkaline electrolyte,the capacitance of RGO-bonded electrodes can reach to 216 F g-1,while that of PTFE-bonded electrodes are only 105 F g-1.Besides,the wettability and performance of RGO-bonded electrodes can be optimized by adjusting thermal reduction temperature.With the decreasing of the thermal reduction temperature,the wettability and the capacitance of the RGO-bonded electrodes gradually increased.With the increasing of the thermal reduction temperature,the conductivity and rate performance of RGO-bonded electrodes gradually increased.As a novel binder,good hydrophilicity of RGO can improve the overall wettability of electrodes.Besides,the developed three-dimensional structure fabricated by RGO can further increase surface area of electrodes and provide a developed channel for fast ion transport.Then the electrochemical performance of electrodes has been improved.